Start with the size chart for the particular glove. Held charts use hand circumference and length from the wrist crease to the middle fingertip. Some models also offer longer or shorter fingers. A size number alone therefore does not describe the whole cut.
Then consider riding conditions. For example, REV’IT! Hydra 3 H2O gloves for colder urban rides combine a waterproof membrane, fleece lining and insulation on the back of the hand. This illustrates why membrane, lining and insulation should be read as separate features in a specification.
When trying gloves on, fasten the wrist closure and hold a handlebar grip. Check finger movement, reach to the levers and the meeting point with your jacket sleeve. The aim is a glove that suits your hand and intended riding; colour and styling can come afterwards.